Methods and compositions for improved electrophoretic display performance

ABSTRACT

The invention is directed to novel methods and compositions useful for improving the performance of electrophoretic displays. The methods comprise adding a high absorbance dye or pigment, or conductive particles, or a conductive filler in the form of nanoparticles and having a volume resistivity of less than about 10 4  ohm cm, or a charge transport material into an electrode protecting layer of the display.

This application is a divisional application of U.S. application Ser.No. 10/785,644, filed Feb. 23, 2004 now U.S. Pat. No. 7,347,957; whichis a continuation-in-part of U.S. application Ser. No. 10/618,257, filedJul. 10, 2003 now abandoned; which claims the benefit of U.S.Provisional Application No. 60/396,680, filed Jul. 17, 2002; thecontents of all the above applications are incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is directed to novel methods and compositions useful forimproving the performance of electrophoretic displays.

2. Description of Related Art

The electrophoretic display (EPD) is a non-emissive device based on theelectrophoresis phenomenon of charged pigment particles suspended in asolvent. It was first proposed in 1969. The display usually comprisestwo plates with electrodes placed opposing each other, separated byspacers. One of the electrodes is usually transparent. Anelectrophoretic fluid composed of a colored solvent with charged pigmentparticles dispersed therein is enclosed between the two plates. When avoltage difference is imposed between the two electrodes, the pigmentparticles migrate to one side or the other causing either the color ofthe pigment particles or the color of the solvent being seen from theviewing side.

There are several different types of EPDs. In the partition type EPD(see M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev.,26(8):1148-1152 (1979)), there are partitions between the two electrodesfor dividing the space into smaller cells in order to prevent undesiredmovement of particles, such as sedimentation. The microcapsule type EPD(as described in U.S. Pat. Nos. 5,961,804 and 5,930,026) has asubstantially two dimensional arrangement of microcapsules each havingtherein an electrophoretic composition of a dielectric fluid and asuspension of charged pigment particles that visually contrast with thedielectric solvent. Another type of EPD (see U.S. Pat. No. 3,612,758)has electrophoretic cells that are formed from parallel line reservoirs.The channel-like electrophoretic cells are covered with, and inelectrical contact with, transparent conductors. A layer of transparentglass from which side the panel is viewed overlies the transparentconductors.

An improved EPD technology was disclosed in co-pending applications,U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000 (corresponding to WO01/67170), U.S. Ser. No. 09/606,654, filed on Jun. 28, 2000(corresponding to WO 02/01281) and U.S. Ser. No. 09/784,972, filed onFeb. 15, 2001 (corresponding to WO02/65215), all of which areincorporated herein by reference. The improved EPD cells are prepared bymicroembossing a layer of thermoplastic or thermoset resin compositioncoated on a first substrate layer to form the microcups of well-definedshape, size and aspect ratio. The microcups are then filled with anelectrophoretic fluid and sealed with a sealing layer. A secondsubstrate layer is laminated over the filled and sealed microcups,preferably with an adhesive layer.

To reduce irreversible electrodeposition of dispersion particles orother charged species onto the electrodes (such as ITO), a thinprotection or release layer may be coated on the electrodes. Theprotective layer improves the performance of the display, including anincrease in display image uniformity and longevity. In addition, afaster electro-optical response has been observed in displays with aprotective layer.

However, the thin protective layer method also has disadvantages. Forexample, the use of a protection or release layer on electrodes tends toresult in deterioration in contrast ratio and bi-stability of the EPD. Ahigher Dmin (or a lower degree of whiteness or % reflectance) in thebackground particularly at low driving voltages is also typicallyobserved in EPDs with coated electrodes.

Accordingly, there is a need for more effective methods to improve theresponse rate and image uniformity and also to reduce irreversibleelectrodeposition of dispersion particles or other charged species ontothe electrodes.

SUMMARY OF THE INVENTION

The present invention relates to novel methods and compositions forimproving the performance of an electrophoretic display.

The first aspect of the present invention is directed to a method forimproving the performance of an electrophoretic display, which methodcomprises adding a high absorbance dye or pigment to at least oneelectrode protecting layer in the display. The term “high absorbance” isdefined as an extinction coefficient greater than 10³ cm⁻¹M⁻¹,preferably in the range of about 10³ to about 5×10⁵ cm⁻¹M⁻¹ and morepreferably in the range of about 5×10³ to about 5×10⁴ cm⁻¹M⁻¹.

The second aspect of the present invention is directed to a method forimproving the performance of an electrophoretic display, which methodcomprises adding conductive particles to at least one electrodeprotecting layer in the display.

The third aspect of the invention is directed to a method for improvingthe performance of an electrophoretic display which method comprisesadding a conductive filler in the form of nanoparticles and having avolume resistivity of less than about 10⁴ ohm cm, preferably about 10²to about 10³ ohm cm, into a composition for the formation of at leastone electrode protecting layer in the display.

The conductive filler is in the form of nanoparticles. The term“nanoparticles”, in the context of the present invention, refers toparticles having an average primary particle size which is smaller thanthe range of UV-visible scattering light (about 0.15 to about 0.3 um) ora typical short-range surface roughness (about 0.05 to about 0.1 um) ofa plastic film. More specifically, the average size of the primaryconductive filler particles suitable for the present invention is in therange of about 5 to about 150 nanometer, preferably about 10 to about 50nanometer and more preferably about 15 to about 20 nanometer. The term“primary particles”, in the context of the present invention, refers tothe particles that can be recognized individually by, for example,electronic or optical microscope. The primary particle size, in thecontext of the present invention, refers to the size of the primaryparticles before flocculation or coagulation.

The fourth aspect of the present invention is directed to a method forimproving the performance of an electrophoretic display, which methodcomprises adding a charge transport material to at least one electrodeprotecting layer in the display.

The fifth aspect of the present invention is directed to an adhesivecomposition comprising an adhesive layer forming material and one ormore of the following: a high absorbance dye or pigment, or conductiveparticles, or a conductive filler in the form of nanoparticles andhaving a volume resistivity of less than about 10⁴ ohm cm, preferablyabout 10² to about 10³ ohm cm, or a charge transport material.

The sixth aspect of the present invention is directed to a sealingcomposition comprising a sealing layer forming material and one or moreof the following: a high absorbance dye or pigment, or conductiveparticles, or a conductive filler in the form of nanoparticles andhaving a volume resistivity of less than about 10⁴ ohm cm, preferablyabout 10² to about 10³ ohm cm, or a charge transport material.

The seventh aspect of the present invention is directed to a primerlayer composition comprising a primer layer forming material and one ormore of the following: a high absorbance dye or pigment, or conductiveparticles, or a conductive filler in the form of nanoparticles andhaving a volume resistivity of less than about 10⁴ ohm cm, preferablyabout 10² to about 10³ ohm cm, or a charge transport material.

When a conductive filler in the form of nanoparticles and having avolume resistivity of less than about 10⁴ ohm cm, preferably about 10²to about 10³ ohm cm, is added to the adhesive, sealing or primer layercomposition, an adhesive, sealing or primer layer having an intendedvolume resistivity of about 10⁷ to 10¹⁰ ohm cm may be achieved.

The adhesive, sealing and primer layer compositions of the presentinvention are particularly useful for electrophoretic displays preparedby the microcup technology.

The eighth aspect of the present invention is directed to a method forimproving the performance of an electrophoretic display by incorporatingnon-light-absorbing conducting particles into a composition for theformation of an electrode protecting layer.

The ninth aspect of the present invention is directed to the use of ahigh absorbance dye or pigment, or conductive particles, or a conductivefiller in the form of nanoparticles and having a volume resistivity ofless than about 10⁴ ohm cm, preferably about 10² to about 10³ ohm cm, ora charge transport material, for improving performance of anelectrophoretic display.

The tenth aspect of the present invention is directed to anelectrophoretic display comprising at least one electrode protectinglayer formed of a composition comprising one or more of the following: ahigh absorbance dye or pigment, or conductive particles, or a conductivefiller in the form of nanoparticles and having a volume resistivity ofless than about 10⁴ ohm cm, preferably about 10² to about 10³ ohm cm, ora charge transport material.

The electrophoretic displays of the present invention show an increasein contrast ratio and image bistability even at low driving voltageswithout trade-off in display longevity and image uniformity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic depiction of an electrophoretic displaycell prepared by the microcup technology.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

Unless defined otherwise in this specification, all technical terms areused herein according to their conventional definitions as they arecommonly used and understood by those of ordinary skill in the art.

The term “microcup” refers to the cup-like indentations which may becreated by methods such as microembossing or a photolithographic processas described in the co-pending application, U.S. Ser. No. 09/518,488.

The term “well-defined”, when describing the microcups or cells, isintended to indicate that the microcup or cell has a definite shape,size and aspect ratio which are pre-determined according to the specificparameters of the manufacturing process.

The term “aspect ratio” is a commonly known term in the art ofelectrophoretic displays. In this application, it refers to the depth towidth or depth to length ratio of the microcups.

The term “Dmax” refers to the maximum achievable optical density of thedisplay.

The term “Dmin” refers to the minimum optical density of the displaybackground.

The term “contrast ratio” refers to the ratio of the reflectance (% oflight reflected) of the Dmin state to the reflectance of the Dmax state.

The term “charge transport material” is defined as a material capable oftransporting either electrons or holes from one side (such as theelectrode side) of the protecting layer to the other side (such as theelectrophoretic fluid side) or vise-versa. Electrons are injected fromthe cathode and holes are injected from the anode into the electrontransporting and hole transporting layer, respectively. A general reviewof the charge transport materials may be found in references, such as P.M. Borsenberger and D. S. Weiss, “Photoreceptors: OrganicPhotoconductors” in “Handbook of Imaging Materials”, A. S. Diamond ed.,pp 379, (1991), Marcel Dekker, Inc.; H. Sher and E W Montroll, Phys.Rev., B12, 2455 (1975); S. A. Van Slyke et. al., Appl. Phys. Lett., 69,2160, (1996); or F. Nuesch et.al., J. Appl. Phys., 87, 7973 (2000).

The term “electrode protecting layer” is defined in the section below.

General Description of the Microcup Technology

FIGS. 1A and 1B depict typical display cells prepared by the microcuptechnology as disclosed in WO01/67170. The microcup based display cell(10) is sandwiched between a first electrode layer (11) and a secondelectrode layer (12). A thin protective layer (13) is optionally presentbetween the cell (10) and the second electrode layer (12) as seen in thefigures. As shown in FIG. 1A, the layer (13) may be a primer layer(adhesion promoting layer) to improve the adhesion between the microcupmaterial and the second electrode layer (12). Alternatively the layer(13) may be a thin layer of the microcup material (as shown in FIG. 1B)if the microcup array is prepared by an embossing process. The cell (10)is filled with an electrophoretic fluid and sealed with a sealing layer(14) on the open side of the microcups. The first electrode layer (11)is laminated onto the sealed cell, preferably with an adhesive (15).

In the context of the present invention, the term “electrode protectinglayer” may be the primer layer or the thin microcup layer (13), sealinglayer (14) or adhesive layer (15) as shown in FIGS. 1A and 1B.

In case of in-plane switching EPDs, one of the electrode layers (11 or12) may be replaced by an insulating layer.

The display panel may be prepared by microembossing or photolithographyas disclosed in WO01/67170. In the microembossing process, an embossablecomposition is coated onto the conductor side of the second electrodelayer (12) and embossed under pressure to produce the microcup array. Toimprove the mold release property, the conductor layer may be pretreatedwith a thin primer layer (13) before coating the embossable composition.

The embossable composition may comprise a thermoplastic or thermosetmaterial or a precursor thereof, such as multifunctional vinylsincluding, but are not limited to, acrylates, methacrylates, allyls,vinylbenzenes, vinylethers, multifunctional epoxides and oligomers orpolymers thereof, and the like. Multifunctional acrylate and oligomersthereof are the most preferred. A combination of a multifunctionalepoxide and a multifunctional acrylate is also very useful to achievedesirable physico-mechanical properties. A low Tg binder orcrosslinkable oligomer imparting flexibility, such as urethane acrylateor polyester acrylate, is usually also added to improve the flexureresistance of the embossed microcups. The composition may contain anoligomer, a monomer, additives and optionally a polymer. The Tg (glasstransition temperature) for the embossable composition usually rangesfrom about −70° C. to about 150° C., preferably from about −20° C. toabout 50° C.

The microembossing process is typically carried out at a temperaturehigher than the Tg. A heated male mold or a heated housing against whichthe mold presses may be used to control the microembossing temperatureand pressure.

The mold is released during or after the embossable composition ishardened to reveal an array of microcups (10). The hardening of theembossable composition may be accomplished by cooling, solventevaporation, cross-linking by radiation, heat or moisture. If the curingof the embossable composition is accomplished by UV radiation, UV mayradiate onto the embossable composition through the transparentconductor layer. Alternatively, UV lamps may be placed inside the mold.In this case, the mold must be transparent to allow the UV light toradiate through the pre-patterned male mold on to the embossablecomposition.

The composition of the primer layer is at least partially compatiblewith the embossing composition or the microcup material after curing. Inpractice, it may be the same as the embossing composition.

In general, the dimension of each individual microcup may be in therange of about 10² to about 1×10⁶ μm2, preferably from about 10³ toabout 1×10⁵ μm². The depth of the microcups is in the range of about 3to about 100 microns, preferably from about 10 to about 50 microns. Theratio between the area of opening to the total area is in the range offrom about 0.05 to about 0.95, preferably from about 0.4 to about 0.9.The width of the openings usually are in the range of from about 15 toabout 450 microns, preferably from about 25 to about 300 microns, fromedge to edge of the openings.

The microcups are then filled with an electrophoretic fluid and sealedas disclosed in U.S. Pat. No. 6,930,818 (corresponding to WO 01/67170),U.S. Pat. No. 6,795,138 (corresponding to WO02/56097), U.S. Pat. No.6,672,921 (corresponding to WO 02/01281) and U.S. Pat. No. 6,933,098(corresponding to WO02/65215), all of which are incorporated herein byreference.

The sealing of the microcups may be accomplished in a number of ways.Preferably, it is accomplished by overcoating the filled microcups witha sealing composition comprising a solvent and a sealing layer formingmaterial selected from the group consisting of thermoplastic elastomers,polyvalent acrylate or methacrylate, cyanoacrylates, polyvalent vinylincluding vinylbenzene, vinylsilane, vinylether, polyvalent epoxide,polyvalent isocyanate, polyvalent allyl, oligomers or polymerscontaining crosslinkable functional groups and the like. Additives suchas a polymeric binder or thickener, photoinitiator, catalyst,vulcanizer, filler, colorant or surfactant may be added to the sealingcomposition to improve the physico-mechanical properties and the opticalproperties of the display. The sealing composition is incompatible withthe electrophoretic fluid and has a specific gravity no greater thanthat of the electrophoretic fluid. Upon solvent evaporation, the sealingcomposition forms a conforming seamless seal on top of the filledmicrocups. The sealing layer may be further hardened by heat, radiationor other curing methods. Sealing with a composition comprising athermoplastic elastomer is particularly preferred. Examples ofthermoplastic elastomers may include, but are not limited to, tri-blockor di-block copolymers of styrene and isoprene, butadiene orethylene/butylene, such as the Kraton™ D and G series from KratonPolymer Company. Crystalline rubbers such aspoly(ethylene-co-propylene-co-5-methylene-2-norbornene) and other EPDM(ethylene propylene diene rubber terpolymer) from Exxon Mobil have alsobeen found very useful.

Alternatively, the sealing composition may be dispersed into anelectrophoretic fluid and filled into the microcups. The sealingcomposition is incompatible with the electrophoretic fluid and islighter than the electrophoretic fluid. Upon phase separation, thesealing composition floats to the top of the filled microcups and formsa seamless sealing layer thereon after solvent evaporation. The sealinglayer may be further hardened by heat, radiation or other curingmethods.

The sealed microcups finally are laminated with the first electrodelayer (11) which may be pre-coated with an adhesive layer (15).

Preferred materials for the adhesive layer may be formed from oneadhesive or a mixture thereof selected from the group consisting ofpressure sensitive, hot melt and radiation curable adhesives. Theadhesive layer forming materials may include, but are not limited to,acrylics, styrene-butadiene copolymers, styrene-butadiene-styrene blockcopolymers, styrene-isoprene-styrene block copolymers, polyvinylbutyral,cellulose acetate butyrate, polyvinylpyrrolidone, polyurethanes,polyamides, ethylene-vinylacetate copolymers, epoxides, multifunctionalacrylates, vinyls, vinylethers, and their oligomers, polymers andcopolymers. Adhesives comprising polymers or oligomers having a highacid or base content such as polymers or copolymers derived from acrylicacid, methacrylic acid, itaconic acid, maleic anhydride, vinylpyridineand derivatives thereof are particularly useful. The adhesive layer maybe post cured by, for example, heat or radiation such as UV afterlamination.

Embodiments of the Present Invention

The term “electrode protecting layer”, as stated above, may be theprimer layer (13), sealing layer (14) or adhesive layer (15) as shown inFIGS. 1A and 1B.

The primer layer (13) of the display, as stated above, may be formedfrom a composition comprising a primer layer forming material such as athermoplastic or thermoset material or a precursor thereof. Examples ofprimer layer forming materials include, but are not limited to,multifunctional acrylates or methacrylates, vinylbenzenes, vinylethers,epoxides or oligomers and polymers thereof. A multifunctional acrylateand oligomers thereof are usually preferred. The thickness of the primerlayer is in the range of about 0.1 to about 5 microns, preferably about0.1 to about 1 microns.

The sealing layer (14) is formed from a composition comprising a solventand a sealing layer forming material selected from the group consistingof thermoplastic elastomers, polyvalent acrylate or methacrylate,cyanoacrylates, polyvalent vinyl including vinylbenzene, vinylsilane,vinylether, polyvalent epoxide, polyvalent isocyanate, polyvalent allyl,oligomers or polymers containing crosslinkable functional groups, andthe like. The thickness of the sealing layer is in the range of about0.5 to about 15 microns, preferably about 1 to about 8 microns.

Materials suitable for the adhesive layer (15) may include, but are notlimited to, acrylics, styrene-butadiene copolymers,styrene-butadiene-styrene block copolymers, styrene-isoprene-styreneblock copolymers, polyvinylbutyral, cellulose acetate butyrate,polyvinylpyrrolidone, polyurethanes, polyamides, ethylene-vinylacetatecopolymers, epoxides, multifunctional acrylates, vinyls, vinylethers,and their oligomers, polymers and copolymers. The thickness of theadhesive layer is in the range of about 0.2 to about 15 microns,preferably about 1 to about 8 microns.

The first aspect of the present invention is directed to a method forimproving the performance of an electrophoretic display, which methodcomprises adding a high absorbance dye or pigment into at least one ofthe electrode protecting layers of the display. The dye or pigment maybe dissolved or dispersed in the electrode protecting layer.

The dye or pigment may be present in more than one electrode protectinglayers on the non-viewing side of the display. If the dye or pigment isused in the primer or the microcup layer, it should not interfere withthe hardening or mold release in the microembossing process.

In addition to the improvement in switching performance, the use of ahigh absorbance dye or pigment in the layer(s) opposite from the viewingside of the display also provides a dark background color and anenhanced contrast ratio.

The dye or pigment preferably has an absorption band in the range of320-800 nm, more preferably 400-700 nm. Suitable dyes or pigments forthe present invention may include, but are not limited to, metalphthalocyanines or naphthalocyanines (wherein the metal may be Cu, Al,Ti, Fe, Zn. Co, Cd, Mg, Sn, Ni, In, Ti, V or Pb), metal porphines(wherein the metal may be Co, Ni or V), azo (such as diazo or polyazo)dyes, squaraine dyes, perylene dyes and croconine dyes. Other dyes orpigments which may generate or transport charge in their excited stateor ground state would also be suitable. Examples of this type of dyes orpigments are those typically used as charge generating materials inorganic photoconductors (See P. M. Borsenberger and D. S. Weiss,“Photoreceptors: Organic Photoconductors” in “Handbook of ImagingMaterials”, A. S. Diamond ed., pp 379, (1991), Marcel Dekker, Inc).

Particularly preferred dyes or pigments are:

-   Cu phthalocyanines and naphthalocyanines such as ORASOL®    (anthraquinone) Blue GN (Color Index C.I.)-   Solvent Blue 67, Cu    {29H,31H-phthalocyaninato(2-)-N29,N30,N31,N32}-{{3-(1-methyethoxy)propyl}amino}sulfonyl    derivative from Ciba Specialty Chemicals (High Point, N.C.);-   Ni phthalocyanine;-   Ti phthalocyanine;-   Ni tetraphenylporphine;-   Co phthalocyanine,-   Metal porphine complexes such as tetraphenylporphine vanadium(IV)    oxide complex and alkylated or alkoxylated derivatives thereof;-   ORASOL® (anthraquinone) Black RLI (C.I. Solvent Black 29, 1:2 Chrome    complex, from Ciba Specialty Chemicals);-   Diazo or polyazo dyes including Sudan dyes such as Sudan Black B    (Color Index C.I. 26150, Fat Black HB, Solvent Black 3), Sudan Blue    (C.I. 61552, Atlasol Blue 2N,    1,4-bis(ethylamino)-9,10-anthraquinone), Sudan R, Sudan Red B (C.I.    26110); Sudan Red 7B (C.I. 26050, Fat Red 7B, Solvent Red 19), Sudan    Yellow or Sudan I (C.I. 12055, 1-phenylazo-2-naphthol, Solvent    Yellow 14), Sudan II (C.I. 12140, Solvent Orange 7), Sudan III (C.I.    26110, Solvent Red 23), or Sudan IV (C.I. 26105, Scarlet Red,    Solvent Red 24);-   Squaraine and croconine dyes such as    1-(4-dimethylamino-pheny)-3-(4-dimethylimmonium-cyclohexa-2,5-dien-1-ylidene)-2-oxo-cyclobuten-4-olate,    1-(4-methyl-2-morpholino-selenazo-5-yl)-3-(2,5-dihydro-4-methy-2[morpholin-1-ylidene-onium]-selenzaol-5-ylidene)-2-oxo-cyclobuten-4-olate    or    1-(2-dimethylamino-4-phenyl-thiazol-5-yl)-3-(2,5-dihydro-2-dimethylimmonium-4-phenyl)-thiazol-5-ylidene)-2-oxo-cyclobuten-4-olate;    and-   Condensed perylene dyes or pigments such as    2,9-di(2-hydroxyethyl)-anthra[2.1,9-def:6,5,10-d′e′f′]diisoquinoline-1,3,8,10-tetrone,    9-di(2-methoxyethyl)-anthra[2.1,9-def:6,5,10-d′e′f′]diisoquinoline-1,3,8,10-tetrone,    bisimidazo[2,1-a:2′,1′-a′]anthra[2.1,9-def:6,5,10-d′e′f′]diisoquinoline-dione    or    anthra[2″,1″,9″:4,5,6:6″,5″,10″:4′,5′,6′]-diisoquinoline[2,1-a:2″1″-a]diperimidine-8,20-dione.

Some of the dyes or pigments such as metal (particularly Cu and Ti)phthalocyanines and naphthalocyanines have also been found useful ascharge transport materials.

The concentration of the dye or pigment may range from about 0.1% toabout 30%, preferably from about 2% to about 20%, by weight of the totalsolid content of the layer. Other additives such as surfactants,dispersion aids, thickeners, crosslinking agents, vulcanizers,nucleation agents or fillers may also be added to enhance the coatingquality and display performance.

The second aspect of the invention is directed to a method for improvingperformance of an electrophoretic display, which method comprises addingparticles of a conductive material into at least one of the electrodeprotecting layers.

The conductive materials include, but not limited to, organic conductingcompounds or polymers, carbon black, carbonaceous particles, graphite,metals, metal alloys or conductive metal oxides. Suitable metals includeAu, Ag, Cu, Fe, Ni, In, Al and alloys thereof. Suitable metal oxides mayinclude indium-tin-oxide (ITO), indium-zinc-oxide (IZO), antimony-tinoxide (ATO), barium titanate (BaTiO₃) and the like. Suitable organicconducting compounds or polymers may include, but are not limited to,poly(p-phenylene vinylene), polyfluorene,poly(4,3-ethylenedioxythiophene), poly(1,2-bis-ethylthio-acetylene),poly(1,2-bis-benzylthio-acetylene),5,6,5′,6′-tetrahydro-[2,2′]bi[1,3]dithiolo[4,5-b][1,4]dithiinylidene],4,5,6,7,4′,5′,6′,7′-octahydro-[[2,2′]bi[benzo[1,3]dithiolylidene,4,4′-diphenyl-[2,2′]bi[1,3]dithiolylidene,2,2,2′,2′-tetraphenyl-bi-thiapyran-4,4′-diylidene,hexakis-bezylthio-benzene and derivatives thereof.

Organic and inorganic particles overcoated with any of theabove-mentioned conductive materials are also useful.

Addition of the conductive material, in the form of particles, into anelectrode protecting layer improves the contrast ratio at low operatingvoltages. However, the amount of the conductive material added should bewell controlled so that it does not cause short or current leakage. Theamount of the conductive material added preferably is in the range offrom about 0.1% to about 40%, more preferably from about 5% to about30%, by weight of the total solid content of the layer.

Additives such as dispersion agents, surfactants, thickeners,crosslinking agents, vulcanizers or fillers may also be added to improvethe coating quality and display performance. The conductive material maybe added to more than one electrode protecting layer. The particle sizeof the conductive material is in the range of from about 0.01 to about 5μm, preferably from about 0.05 to about 2 μm.

The third aspect of the invention is directed to a method for improvingthe performance of an electrophoretic display which method comprisesadding a conductive filler in the form of nanoparticles and having avolume resistivity of less than about 10⁴ ohm cm, preferably about 10²to about 10³ ohm cm, into a composition for the formation of at leastone electrode protecting layer in the display.

The conductive filler is in the form of nanoparticles. The term“nanoparticles”, in the context of the present invention, refers toparticles having an average primary particle size which is smaller thanthe range of UV-visible scattering light (about 0.15 to about 0.3 um) ora typical short range surface roughness (about 0.05 to about 0.1 um) ofa plastic film. More specifically, the average size of the primaryconductive filler particles suitable for the present invention is in therange of about 5 to about 150 nanometer, preferably about 10 to about 50nanometer and more preferably about 15 to about 20 nanometer.

The resulting electrode protecting layer may have a desired volumeresistivity in the range of about 10⁷ to 10¹⁰ ohm cm. The primer layer,sealing layer or adhesive layer forming material in the composition mayhave a volume resistivity in the range of 10¹² to 10¹⁴ ohm cm.

Suitable conductive fillers may include, but are not limited to,conductive metal oxide particles, carbon black, graphite, carbonnanotube, conductive polymers such as polythiophene (PT), polyacetylene,polypyrrole (PPy) or polyaniline (PAN), metal particles or flakes suchas silver particles or flakes and conductive nanoclusters such as Au orCu nanoclusters. Specific examples of suitable conductive fillers mayinclude zinc antimonate, zinc sulfide, indium tin oxide, antimony tinoxide and the like.

The conductive filler in the form of nanoparticles may be dispersed in asol gel (which is a colloidal dispersion of the nanoparticles). In thiscase, the sol gel may comprise a solvent compatible with the embossablecomposition. For example, the solvent may be 2-butanone, acetone,isopropanol or the like. The concentration of the conductive filler inthe sol gel may be in the range of about 15% to about 45%, preferably inthe range of about 30% to about 40%.

The conductive filler preferably is colorless and highly transparent.For example, it should have about 75% to about 95%, preferably about 85%to about 90%, transmission in the visible light range for a 20 μm driedfilm containing about 30% by weight of the conductive filler.

In one embodiment, the conductive filler is Celnax® (from NissanChemical) which is zinc antimonate colloidal nanoparticles.

The fourth aspect of the invention is directed to a method for improvingthe performance of an electrophoretic display, which method comprisesadding a charge transport material to at least one of the electrodeprotecting layers of the display.

Charge transport materials are materials capable of transporting eitherelectrons or holes from one side (such as the electrode side) of theelectrode protecting layer to the other side (such as theelectrophoretic fluid side) or vice-versa. Electrons are injected fromthe cathode and holes are injected from the anode into the electrontransporting and hole transporting layers, respectively. A generalreview of the charge transport materials may be found in references,such as P. M. Bosenberger and D. S. Weiss, “Photoreceptors: OrganicPhotoconductors” in “Handbook of Imaging Materials”, A. S. Diamond ed.,pp 379, (1991), Marcel Dekker, Inc.; H. Sher and E W Montroll, Phys.Rev., B12, 2455 (1975); S. A. Van Slyke et.al., Appl. Phys. Lett., 69,2160, (1996); or F. Nuesch et.al., J. Appl. Phys., 87, 7973 (2000).

Suitable electron and hole transport materials may be found from generaltechnical reviews in organic photoconductors and organic light emittingdiodes such as those listed above.

The hole transport materials are typically compounds having a lowionization potential which may be estimated from their solutionoxidation potentials. In the context of the present invention, compoundshaving an oxidation potential less than 1.4 V, particularly less than0.9 V (vs SCE) are found useful as the charge transport materials.Suitable charge transport materials should also have acceptable chemicaland electrochemical stability, reversible redox behavior and sufficientsolubility in the protection layer for the electrodes. Too low anoxidation potential may result in undesirable oxidation in air and ashort display shelf life. Compounds having oxidation potentials between0.5-0.9 V (vs SCE) are found particularly useful for this invention.

In the context of the present invention, particularly useful holetransport materials include compounds in the general classes of:

-   Pyrazolines such as    1-phenyl-3-(4′-dialkylaminostyryl)-5-(4″-dialkylaminophenyl)pyrazoline;-   Hydrazones such as p-dialkylaminobenzaldehyde-N,N-diphenylhydrazone,    9-ethyl-carbazole-3-aldehyde-N-methyl-N-phenylhydrazone,    pyrene-3-aldehyde-N,N-diphenylhydrazone,    4-diphenylamino-benzaldehyde-N,N-diphenylhydrazone,    4-N,N-bis(4-methylphenyl)-amino-benzaldehyde-N,N-diphenylhydrazone,    4-dibenzylamino-benzaldehyde-N,N-diphenylhydrazone or    4-dibenzylamino-2-methyl-benzaldehyde-N,N-diphenylhydrazone;-   Oxazoles and oxadiazoles such as    2,5-bis-(4-dialkylaminophenyl)-4-(2-chlorophenyl)oxazole,    2,5-bis-(4-N,N′-dialkylaminophenyl)-1,3,4-oxadiazole,    2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,2,3-oxadiazole,    2,2′-(1,3-phenylene)bis[5-[4-(1,1-dimethylethyl)phenyl]1,3,4-oxadiazole,    2,5-bis(4-methylphenyl)-1,3,4-oxadiazole or    1,3-bis(4-(4-diphenylamino)-phenyl-1,3,4-oxadiazol-2-yl)benzene;-   Enamines, carbazoles or arylamines, particularly triaryamines such    as bis(p-ethoxyphenyl)acetaldehyde di-p-methoxyphenylamine enamine,    N-alkylcarbazole, trans-1,2-biscarbazoyl-cyclobutane,    4,4′-bis(carbazol-9-yl)-biphenyl,    N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1-bi[phenyl]-4,4′-diamine,    4,4′-bis(N-naphthyl-N-phenyl-amino)biphenyl (or    N,N′-di(naphthalene-2-yl)-N,N′-diphenyl-benzidine);    4,4′,4″-trismethyl-triphenylamine,    N-biphenylyl-N-phenyl-N-(3-methylphenyl)amine,    4-(2,2-bisphenyl-ethen-1-yl)triphenylamine,    N,N′-di-(4-methyl-pheny)N,N′-diphenyl-1,4-phenylendiamine,    4-(2,2-bisphenyl-ethen-1-yl)-4′,4″-dimethyl-triphenylamine,    N,N,N′N′-tetraphenylbenzidine,    N,N,N′,N′-tetrakis(4-methyphenyl)-benzidine,    N,N′-bis-(4-methylphenyl)-N,N′-bis-(phenyl)-benzidine,    4,4′-bis(dibenz-azepin-1-yl)biphenyl;    4,4″-bis(dihydro-dibenz-azepin-1-yl)-biphenyl,    di-(4-dibenzylamino-phenyl)-ether,    1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)cyclohexane,    4,4′-bis(N,N-diphenylamino)-quaterphenyl,    N,N,N′,N′-tetrakis(naphtha-2-yl)benzidine,    N,N′-bis(phenanthren-9-yl)-N,N′-bis-phenyl-benzidine,    N,N′-bis(phenanthren-9-yl)-N,N′-bis-phenyl-benaidine,    4,4′,4″-tris(carbazol-9-yl)-triphenylamine,    4,4′,4″-tris(N,N-diphenylamino)-triphenylamine,    4,4′-bis(N-(1-naphthyl)-N-phenyl-amino)-quaterphenyl,    4,4′,4″-tris(N-(1-naphthyl)-N-phenyl-amino) triphenylamine or    N,N′-diphenyl-N,N′-bis(4′-(N,N-bis(naphthy-1-yl)-amino)-biphenyl-4-yl)-benzidine;-   Triarylmethanes such as    bis(4-N,N-dialkylamino-2-methylphenyl)-phenylmethane;-   Biphenyls such as 4,4′-bis(2,2-diphenyl-ethen-1-yl)-biphenyl;-   Dienes and dienones such as 1,1,4,4-tetraphenyl-butadiene,    4,4′-(1,2-ethanediylidene)-bis(2,6-dimethyl-2,5-cyclohexadien-1-one),    2-(1,1-dimethylethyl)-4-[3-(1,1-dimethylethyl)-5-methyl-4-ox-2,5-cyclohexa-dien-1-ylidene]-6-methy-2,5-cyclohexadien-1-one,    2,6-bis(1,1-dimethylethyl)4-[3,5-bis(1,1-dimethylethyl)4-oxo-2,5-cyclohexa-dien-1-ylidene]-2,5-cyclohexadien-1-one    or    4,4-(1,2-ethanediylidene)-bis(2,6-(1,1-dimethyl-ethyl)2,5-cyclohexadien-1-one);    and-   Triazoles such as 3,5-bis(4-tert-phenyl)-4-phenyl-triazole or    3-(4-biphenylyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole.

Oligomeric or polymeric derivatives containing any of theabove-mentioned functional groups are also useful as charge transportmaterials.

Particularly useful electron transport materials include electrondeficient compounds in the general classes of:

-   Fluorenones such as 2,4,7-trinitro-9-fluorenone or    2-(1,1-dimethylbutyl)-4,5,7-trinitro-9-fluorenone; and-   Nitriles such as (4-butoxycarbonyl-9-fluorenylidene)malononitrile,    2,6-di-tert-butyl-4-dicyanomethylene-4-H-thiopyran-1,1-dioxide,    2-(4-(1-methyl-ethyl)-phenyl)-6-phenyl-4H-thiopyran-4-ylidene]-propanedinitril-1,1-dioxide    or    2-phenyl-6-methylphenyl-4-dicyanomethylene-4-H-thiopyran-1,1-dioxide    or 7,7,8,8-tetrachcyanonquinodimethane.

The oligomeric or polymeric derivatives containing any of theabove-mentioned functional groups are also useful.

The hole and electron transfer materials may be co-present in the samelayer or even in the same molecule or in different layers on opposite orthe same side of the display cell. Dopants and host materials such as4-(dicyanomethylene)-2-methyl-6-(julolidin-4-yl-vinyl)-4H-pyran,bis(2-2-hydroxyphenyl)-benz-1,3-thiazolato)-Zn complex,bis(2-(2-hydroxyphenyl)-benz-1,3-oxadiazoleato)-Zn complex,tris(8-hydroxy-chinolinato)-Al complex,tris(8-hydroxy-4-methyl-chinolinato)-Al complex ortris(5-chloro-8-hydroxy-chinolinato)-Al complex may also be added intothe electrode protecting layer.

The charge transport material may be incorporated into the compositionof one electrode protecting layer or may be present in more than onelayers. A clear and colorless charge transport material is preferred ifit is to be added into the electrode protecting layer on the viewingside of the display. The concentration of the charge transport materialmay range from about 0.1% to about 30%, preferably from about 2% toabout 20%, by weight of the total solid content of the layer. Otheradditives such as surfactants, dispersion aids, thickeners, crosslinkingagents, vulcanizers, nucleation agents or fillers may also be added toenhance the coating quality and display performance.

It should be noted that the above four aspects of the invention may beperformed alone or in any combination. More than one aspect of theinvention may also be co-present in the same electrode protecting layer.The materials used in the electrode protecting layer on the viewing sideof the display are preferred to be colorless and transparent. Also, thematerials used in the primer and microcup layers should not interferewith the hardening (such as UV curing) of the layers or mold release inthe embossing process.

The fifth aspect of the present invention is directed to an adhesivecomposition comprising an adhesive layer forming material and one ormore of the following: a high absorbance dye or pigment, or conductiveparticles, or a conductive filler in the form of nanoparticles andhaving a volume resistivity of less than about 10⁴ ohm cm, preferablyabout 10² to about 10³ ohm cm, or a charge transport material.

The sixth aspect of the present invention is directed to a sealingcomposition comprising a sealing layer forming material and one or moreof the following: a high absorbance dye or pigment, or conductiveparticles, or a conductive filler in the form of nanoparticles andhaving a volume resistivity of less than about 10⁴ ohm cm, preferablyabout 10² to about 10³ ohm cm, or a charge transport material.

The seventh aspect of the present invention is directed to a primerlayer composition comprising a primer layer forming material and one ormore of the following: a high absorbance dye or pigment, or conductiveparticles, or a conductive filler in the form of nanoparticles andhaving a volume resistivity of less than about 10⁴ ohm cm, preferablyabout 10² to about 10³ ohm cm, or a charge transport material.

The sealing, adhesive and primer layer compositions are particularlyuseful for electrophoretic displays prepared from the microcuptechnology.

In the case of a conductive filler, the composition may be prepared bygradually adding the conductive filler in the form of nanoparticles andhaving a volume resistivity of less than about 10⁴ ohm cm, preferablyabout 10² to about 10³ ohm cm, into a composition under strong stirringfor a period of time until the composition is homogeneously blended. Thelength of the time depends on the stirring conditions. The conductivefiller may be added in a dried form or in the form of a sol gel asdescribed above. The concentration of the conductive filler (calculatedon the basis of the dried form) in the composition for the adhesive,sealing or primer layer may range from about 0.01% to about 50%,preferably from about 15% to about 45%, by weight of the total solidcontent.

Additives such as dispersion agents, surfactants, thickeners,crosslinking agents or vulcanizers may also be added into any of thecompositions discussed above to improve the coating quality and displayperformance.

Suitable adhesive layer forming materials, sealing layer formingmaterials, primer layer forming materials, thermoplastic or thermosetmaterials, high absorbance dyes or pigments, conductive particles,conductive fillers in the form of nanoparticles and having a volumeresistivity of less than about 10⁴ ohm cm, preferably about 10² to about10³ ohm cm and charge transport materials used in the compositions haveall been described in this application.

The eighth aspect of the present invention is directed to a method forimproving the performance of an electrophoretic display by incorporatingnon-light-absorbing conducting particles into a composition for theformation of an electrode protecting layer.

The ninth aspect of the present invention is directed to the use of oneor more of the following: a high absorbance dye or pigment, orconductive particles, or a conductive filler in the form ofnanoparticles and having a volume resistivity of less than about 10⁴ ohmcm, preferably about 10² to about 10³ ohm cm, or a charge transportmaterial for improving performance of an electrophoretic display.

The tenth aspect of the present invention is directed to anelectrophoretic display comprising at least one electrode protectinglayer formed of a composition comprising one or more of the following: ahigh absorbance dye or pigment, or conductive particles, or a conductivefiller in the form of nanoparticles and having a volume resistivity ofless than about 10⁴ ohm cm, preferably about 10² to about 10³ ohm cm, ora charge transport material.

While the microcup technology as disclosed in WO01/67170 is discussed inthis application, it is understood that the methods, compositions anduses of the present invention are applicable to all types ofelectrophoretic displays, including but not limited to, themicrocup-based displays (WO01/67170), the partition type displays (seeM. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26(8):1148-1152(1979)), the microcapsule type displays (U.S. Pat. Nos. 5,961,804 and5,930,026) and the microchannel type displays (U.S. Pat. No. 3,612,758).

EXAMPLES

The following examples are given to enable those skilled in the art tomore clearly understand and to practice the present invention. Theyshould not be considered as limiting the scope of the invention, butmerely as being illustrative and representative thereof.

Comparative Example 1 Example 1A Preparation of Primer CoatedTransparent Conductor Film

A primer coating solution containing 33.2 gm of EB 600™ (acrylated epoxyoligomer, UCB, Smyrna, Ga.), 16.12 gm of SR 399™ (pentafunctionalmonomer, Sartomer, Exton, Pa.), 16.12 gm of TMPTA (trimethylolpropanetriacrylate, UCB, Smyrna, Ga.), 20.61 gm of HDDA (1,6-hexanedioldiacrylate, UCB, Smyrna, Ga.), 2 gm of Irgacure™ 369([2-benzyl-2-(dimethylamino)-1-[4-(4-morpholinyl)phenyl]-1-butanone],Ciba, Tarrytown, N.Y.), 0.1 gm of Irganox™ 1035 (thiodiethylenebis(3,5-di(tert)-butyl-4-hydroxyhydrocinnamate), Ciba), 44.35 gm ofpoly(ethyl methacrylate) (MW. 515,000, Aldrich, Milwaukee, Wis.) and399.15 gm of MEK (methyl ethyl ketone) was mixed thoroughly and coatedonto a 3 mil transparent conductor film (ITO/PET film, 5 mil OC50 fromCPFilms, Martinsville, Va.) using a #4 wire bar. The coated ITO film wasdried in an oven at 65° C. for 10 minutes, then exposed to 1.8 J/cm² ofUV light under nitrogen using a UV conveyer (DDU, Los Angles, Calif.).

Example 1B Preparation of Microcups

33.15 Gm of EB 600™ (UCB, Smyrna, Ga.), 32.24 gm of SR 399™ (Sartomer,Exton, Pa.), 6.00 gm of EB1360™ (silicone acrylate, UCB, Smyrna, Ga.), 8gm of Hycar 1300×43 (reactive liquid polymer, Noveon Inc. Cleveland,Ohio), 0.2 gm of Irgacure™ 369 (Ciba, Tarrytown, N.Y.), 0.04 gram of ITX(Isopropyl-9H-thioxanthen-9-one, Aldrich, Milwaukee, Wis.), 0.1 gm ofIrganox™ 1035 (Ciba, Tarrytown, N.Y.) and 20.61 gram of HDDA(1,6-hexanediol diacrylate, UCB, Smyrna, Ga.) were mixed thoroughly witha Stir-Pak mixer (Cole Parmer, Vernon, Ill.) at room temperature forabout 1 hour, and degassed by centrifuge at 2000 rpm for about 15minutes.

The microcup composition was slowly coated onto a 4″×4″ electroformed Nimale mold for an array of 72 μm (length)×72 μm (width)×35 μm (depth)×13μm (width of top surface of spacing between cups) microcups. A plasticblade was used to remove excess of fluid and gently squeeze it into“valleys” of the Ni mold. The coated Ni mold was heated in an oven at65° C. for 5 minutes and laminated with the primer coated ITO/PET filmprepared in Example 1A, with the primer layer facing the Ni mold using aGBC Eagle 35 laminator (GBC, Northbrook, Ill.) preset at a rollertemperature of 100° C., lamination speed of 1 ft/min and the roll gap at“heavy gauge”. A UV curing station with a UV intensity of 2.5 mJ/cm² wasused to cure the panel for 5 seconds. The ITO/PET film was then peeledaway from the Ni mold at a peeling angle of about 30 degree to give a4″×4″ microcup array on ITO/PET. An acceptable release of the microcuparray from the mold was observed. The thus obtained microcup array wasfurther post-cured with a UV conveyor curing system (DDU, Los Angles,Calif.) with a UV dosage of 1.7 J/cm².

Example 1C Preparation of Electrophoretic Fluid

5.9 Gm of TiO₂ R900™ (DuPont) was added to a solution containing of 3.77gm of MEK, 4.54 gm of N3400™ aliphatic polyisocyanate (Bayer AG) and0.77 gm of 1-[N,N-bis(2-hydroxyethyl)amino]-2-propanol (Aldrich). Theresultant slurry was homogenized for 1 minute at 5-10° C., after which0.01 gm of dibutyltin dilaurate (Aldrich) was added and the mixture washomogenized for an additional minute. Finally a solution containing 20gm of HT-200™ (Ausimont, Thorofare, N.J.) and 0.47 gm of R_(f)-amine4900[a precondensate of Krytox methyl ester (from Du Pont) andtris(2-aminoethyl)amine (Aldrich) prepared as shown below] was added andthe mixture was homogenized again for 3 more minutes at roomtemperature.

The R_(f)-amine4900 was prepared according to the following reaction:

The slurry prepared above was added slowly over 5 minutes at roomtemperature under homogenization into a mixture containing 31 gm ofHT-200 and 2.28 gm of R_(f)-amine4900. The resultant TiO₂ microcapsuledispersion was stirred under low shear with a mechanical stirrer at 35°C. for 30 minutes, then heated to 85° C. to remove MEK and post cure theinternal phase for three hours. The dispersion showed a narrow particlesize distribution ranging from 0.5-3.5 microns. The slurry was dilutedwith equal amount of PFS-2™ (Ausimont, Thorofare, N.J.) and themicrocapsules were separated by centrifuge fractionation to remove thesolvent phase. The solid collected was washed thoroughly with PFS-2™ andredispersed in HT-200.

Example 1D Filling and Sealing with a Sealing Composition

1 Gm of an electrophoretic composition containing 6 parts (based on dryweight) of the TiO₂ microparticles prepared above and 94 parts of aHT-200 (Ausimont) solution of 1.5 wt % of a perfluorinatedCu-phthalocyanine dye (FC-3275, 3M, St. Paul, Minn.) was metered intothe 4″×4″ microcup array prepared from Example 1B. The excess of fluidwas scraped away by a rubber blade. The filled microcups were thenovercoated with a 10% rubber solution consisting of 9 parts of KratonG1650 (Shell, Tex.), 1 part of GRP 6919 (Shell), 3 parts of Carb-O-SilTS-720 (Cabot Corp., Ill.), 78.3 parts of Isopar E and 8.7 parts ofisopropyl acetate by a Universal Blade Applicator and dried at roomtemperature to form a seamless sealing layer of about 2-3 μm drythickness with good uniformity.

Example 1E Lamination

The ITO side of an ITO/PET conductor film (5 mil OC50 from CPFilms) wasovercoated with a 25 wt % solution of a pressure sensitive adhesive(Durotak 1105, National Starch, Bridgewater, N.J.) in methyl ethylketone (MEK) by a Myrad bar (targeted coverage: 0.6 gm/ft²). Theadhesive coated ITO/PET layer was then laminated over the sealedmicrocups prepared from Example 1D with a GBC Eagle 35 laminator at 70°C. The lamination speed was set at 1 ft/min with a gap of 1/32″. Thethus prepared EPD panel showed a contrast ratio of 1.5 at ±20 V againsta black background.

Example 2

The procedure of Example 1 was repeated, except that the sealing layer(Example 1D) and the adhesive layer (Example 1E) were replaced by thoseof Examples 2A and 2B respectively.

Example 2A Sealing Layer Composition Containing Carbon Black

27.8 Gm of carbon black (Vulcan™ XC72, Cabot Corp.) was dispersedthoroughly into 320 gm of an isopropyl acetate/Isopar E ( 1/9) solutioncontaining 0.75 wt % of Disperse-Ayd 6 (Elementis Specialties) using ahigh-speed disperser (Powergen, model 700 equipped with a 20mm-saw-tooth shaft). A 10% (by weight) rubber solution (80 gm)containing 9 parts of Kraton™ G1650, 9 parts of Kraton™ RPG6919 (fromShell Chemical), 1 part of Isopropyl acetate and 81 parts of Isopar-Ewas added into the carbon black dispersion and mixed for another 30minutes. The resultant carbon black dispersion was mixed with anadditional 1780 gm of the same 10% rubber (Kraton™ G1650/Kraton™RPG6919=9/1) solution, homogenized using a Silverson L4RT-A homogenizerfor 2 hours and filtered through a 40 μm filter.

Example 2B Adhesive Layer Composition Containing a Dye

A solution containing of 6.0 gm of a 25 wt % solution of ORASOL®(anthraquinone) BlueGL (C.I. Solvent Blue 70, Chemical Class:Cu-phthalocyanine, Ciba Specialty Chemicals, High Point, N.C.) in MEK,20.0 gm of Duro-Tak™ 80-1105 adhesive (50% solid from National Starch,Bridgewater, N.J.) and 51.0 gm of MEK was coated onto the ITO side of anITO/PET film and laminated onto the sealed microcup array containing theelectrophoretic fluid as prepared in Example 1. The target coverage ofthe adhesive remains the same: 0.6 gm/ft².

The EPD panel showed a contrast ratio of 6.2 at ±20V.

Examples 3 to 7

The procedure of Example 2 was followed in Examples 3 to 7, except thatthe ORASAL® (anthraquinone) Blue GL was replaced with the different dyesin the adhesive layer as shown in Table 1.

TABLE 1 Effect of Dyes and Carbon Black in Adhesive and Sealing LayersAdditive in Adhesive Additive in Contrast Ratio Contrast Ratio LayerSealing Layer at ±20 V at ±30 V Comparative None None 1.5 2.2 Example 1Example 2 13 wt % ORASOL ® 13 wt % Carbon 6.2 9.3 Blue GL (C.I. SolventBlack Blue 70, Cu- phthalocyanine) Example 3 13 wt % ORASOL ® 13 wt %Carbon 6.0 8.5 Red BL (C.I. Solvent Black Red 122; 1:2 chrome complexExample 4 13 wt % ORASOL ® 13 wt % Carbon 5.5 8.2 Yellow 2GLN (C.I.Black Solvent Yellow 88; 1:2 chrome complex Example 5 13 wt % ORASOL ®13 wt % Carbon 5.2 8.1 Black CN (C.I. Solvent Black Black 28; 1:2 chromecomplex) Example 6 13 wt % ORASOL ® 13 wt % Carbon 5.0 7.2 Black RLI(C.I. Solvent Black Black 29; 1:2 chrome complex) Example 7 13 wt %Sudan Black 13 wt % Carbon 5.0 6.7 (C.I. 26150, Fat Black Black HB,Solvent Black 3)

All the ORASOL® dyes in Table 1 were obtained from Ciba SpecialtyChemicals, and the Sudan Black was obtained from Aldrich.

Example 8

The procedure of Example 2 was followed, except that the ORASOL® BlueGLin the adhesive layer was replaced with barium titanate (BaTiO₃). Thus,12 gm of barium titanate (K-Plus-16, from Cabot, Mass.) was dispersedusing a sonicator (Fisher dismembrator, Model 550) into the adhesivesolution containing 15.5 g of Duro-Tak™ 80-1105, 18.8 gm of ethylacetate, 15.9 gm of toluene, 1.4 gm of hexane and 1.1 gm of a polymericdispersant (Disperbyk 163, BYK Chemie). The adhesive was coated onto theITO side of an ITO/PET film (targeted dry coverage: 6 mm) and theresultant film was laminated onto the sealed microcup array as inExample 2 at 100° C.

The EPD panel showed a contrast ratio of 6.1 at ±30V.

Comparative Example 9

The procedure of Example 8 was followed, except that no BaTiO₃ was usedin the adhesive layer (target dry coverage: 6 μm).

The EPD panel showed a contrast ratio of 4.7 at ±30V.

Example 10

The procedure of Example 2 was followed, except that the Orasol™ BlueGLin the adhesive layer was replaced withN,N′-(bis(3-methylphenyl)-N-N′-diphenylbenzidine (BMD). Thus, 0.42 gm ofBMD was dissolved at 80° C. into 28 gm of a 10 wt % solution of adhesiveDuro-Tak™ 80-1105 in dimethyl formamide (DMF). The resultant adhesivesolution was coated on the ITO side of a 5-mil ITO/PET using wire bars#12 and the resultant film was laminated onto the sealed microcup arrayas in Example 2 at 100° C.

The EPD panel showed a contrast ratio of about 3 at ±20V.

Comparative Example 11

The procedure of Example 10 was followed, except that no BMD was used inthe adhesive layer. The EPD panel thus prepared showed a contrast ratioof about 2 at ±20V.

Example 12

The same procedure of Example 1 was followed except that CELNAX®equivalent to 45% by weight of zinc antimonite was added to the primerlayer.

Example 12 showed improvement in image bistability with the addition ofCELNAX® in the primer layer.

While the present invention has been described with reference to thespecific embodiments thereof, it should be understood that variouschanges may be made and equivalents may be substituted without departingfrom the true spirit and scope of the invention. In addition, manymodifications may be made to adapt a particular situation, materials,compositions, processes, process step or steps, to the objective, spiritand scope of the present invention. All such modifications are intendedto be within the scope of the claims appended hereto.

1. A method for improving contrast ratio of an electrophoretic display,wherein said electrophoretic display comprises (i) display cells filledwith an electrophoretic fluid comprising charged pigment particlesdispersed in a solvent or solvent mixture, (ii) a sealing layer on theopen side of each of said display cells, (iii) optionally a primerlayer, and (iv) optionally an adhesive layer; which method comprisesforming at least one of the sealing layer, the primer layer, and theadhesive layer by adding a high absorbance dye or pigment to said layer;wherein the formed electrophoretic display comprises at least one layer(ii-iv) containing said high absorbance dye or pigment.
 2. The method ofclaim 1 wherein said dye or pigment has an absorption band in the rangeof about 320 to about 800 nm.
 3. The method of claim 2 wherein said dyeor pigment has an absorption band in the range of about 400 to about 700nm.
 4. The method of claim 1 wherein said dye or pigment is selectedfrom the group consisting of metal phthalocyanine, metalnaphthalocyanine, metal porphine, azo dyes, squaraine dyes, perylenedyes, and croconine dyes.
 5. The method of claim 4 wherein said metal inmetal phthalocyanine or metal naphthalocyanine is Cu, Al, Ti, Fe, Zn,Co, Cd, Mg, Sn, Ni, In, Ti, V or Pb.
 6. The method of claim 4 whereinsaid metal in metal porphine is Co, Ni or V.
 7. The method of claim 4wherein said azo dye is a diazo or polyazo dye.
 8. The method of claim 1wherein said dye or pigment is a charge generating material used inorganic photoconductors.
 9. The method of claim 1 wherein said dye orpigment is selected from the group consisting of Cu phthalocyanine, Cunaphthalocyanine, C.I. Solvent Blue 67, Ni phthalocyanine, Tiphthalocyanine, Ni tetraphenylporphine, Co phthalocyanine, anthraquinoneblue, anthraquinone red, anthraquinone yellow, anthraquinone black,tetraphenylporphine vanadium(IV) oxide complex and alkylated oralkoxylated derivatives thereof, C.I. Solvent Black 29, Sudan Black B,Sudan Blue, Sudan R, Sudan Yellow, Sudan I, Sudan II, Sudan III, SudanIV,1-(4-dimethylamino-pheny)-3-(4-dimethylimmonium-cyclohexa-2,5-dien-1-ylidene)-2-oxo-cyclobuten-4-olate,1-(4-methyl-2-morpholino-selenazo-5-yl)-3-(2,5-dihydro-4-methy-2[morpholin-1-ylidene-onium]-selenzaol-5-ylidene)-2-oxo-cyclobuten-4-olate,1-(2-dimethylamino-4-phenyl-thiazol-5-yl)-3-(2,5-dihydro-2-dimethylimmonium-4-phenyl)-thiazol-5-ylidene)-2-oxo-cyclobuten-4-olate;2,9-di(2-hydroxyethyl)-anthra[2.1,9-def:6,5,10-d′e′f′]diisoquinoline-1,3,8,10-tetrone,9-di(2-methoxyethyl)-anthra[2.1,9-def:6,5,10-d′e′f′]diisoquinoline-1.3,8,10-tetrone,bisimidazo[2,1-a:2′,1′]anthra[2.1,9-def:6,5,10-d′e′f′]diisoquinoline-dioneandanthra[2″,1″,9″:4,5,6:6″,5″,10″:4′,5′,6]-diisoquinoline[2,1-a:2′1′-a]diperimidine-8,20-dione.10. The method of claim 1 wherein the method comprises forming theprimer layer from a composition comprising a thermoplastic, thermoset ora precursor thereof and the high absorbance dye or pigment.
 11. Themethod of claim 10 wherein said thermoplastic or thermoset is selectedfrom the group consisting of polyvinylbutyrals, cellulose acetatebutyrate, poly(alkyl acrylates), poly(alkyl methacrylates), polyethers,polyurethanes, polyamides, polyesters, polycarbonates, multifunctionalacrylates or methacrylates, vinylbenzenes, vinylethers, epoxides andoligomers or polymers thereof.
 12. The method of claim 1 wherein themethod comprises forming the sealing layer from a composition comprisinga sealing layer forming material and the high absorbance dye or pigment.13. The method of claim 12 wherein said sealing layer forming materialis selected from the group consisting of thermoplastic elastomers,polyvalent acrylate or methacrylate, cyanoacrylates, polyvalent vinyl,polyvalent epoxide, polyvalent isocyanate, polyvalent allyl andoligomers or polymers containing crosslinkable functional groups. 14.The method of claim 1 wherein the method comprises forming the adhesivelayer from a composition comprising an adhesive layer forming materialand the high absorbance dye or pigment.
 15. The method of claim 14wherein said adhesive layer forming material is selected from the groupconsisting of acrylics, styrene-butadiene copolymers,styrene-butadiene-styrene block copolymers, styrene-isoprene-styreneblock copolymers, polyvinylbutyral, cellulose acetate butyrate,polyvinylpyrrolidone, polyurethanes, polyamides, ethylene-vinylacetatecopolymers, epoxides, multifunctional acrylates, vinyls, vinylethers andoligomers, polymers and copolymers thereof.
 16. The method of claim 1wherein said dye or pigment is in the amount of from about 0.1 to about30% by weight of the total solid content of the composition for theadhesive layer, the primer layer, or the sealing layer.
 17. The methodof claim 16 wherein said dye or pigment is in the amount of from about 2to about 20% by weight of the total solid content of the composition forthe adhesive layer, the primer layer, or the sealing layer.
 18. Themethod of claim 1 wherein said dye or pigment has an extinctioncoefficient greater than 10³ cm⁻¹M⁻¹.
 19. The method of claim 1 whereinsaid dye or pigment has an extinction coefficient in the range of about10³ to about 5×10⁵ cm⁻¹M⁻¹.
 20. The method of claim 1 wherein said dyeor pigment has an extinction coefficient in the range of about 5×10³ toabout 5×10⁴ cm⁻¹M⁻¹.